Since many automobile parts have complex shapes, materials for the parts are required to be excellent in terms of formability indexes such as elongation (El), bendability, and stretch flange formability (also referred to as hole-expanding property). In addition, in the case where strength is increased to a TS of 900 MPa class or more, there is a case where very expensive rare chemical elements such as Ti, Nb, V, Cu, Ni, Cr, and Mo are actively added from the viewpoint of achieving high strength.
Here, some techniques have been proposed regarding a galvannealed steel sheet excellent in terms of both elongation and stretch flange formability. For example, Patent Literature 1 discloses a high-strength and high-yield-ratio galvannealed steel sheet having a tensile strength of 980 MPa or more and excellent formability, the steel sheet having a chemical composition containing, by mass %, C: 0.12% to 0.3%, Si: 0.1% or less (exclusive of 0%), Mn: 2.0% to 3.5%, P: 0.05% or less (exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al: 0.005% to 0.1%, N: 0.015% or less (exclusive of 0%), and the balance being Fe and incidental impurities, and having a metallographic structure including a bainite phase as a primary phase, in which the area ratio of a ferrite phase is 3% to 20% and the area ratio of a martensite phase is 10% to 35% with respect to the whole of the microstructure. Patent Literature 2 discloses a high-strength galvanized steel sheet excellent in terms of hole-expanding property and ductility, the steel sheet having a chemical composition containing, by mass %, C: 0.03% to 0.20%, Si: 1.0% or less, Mn: 0.01% to 3%, P: 0.0010% to 0.1%, S: 0.0010% to 0.05%, Al: 0.3% to 2.0%, Mo: 0.01% to 5.0%, one, two, or more of Ti: 0.001% to 0.5%, Nb: 0.001% to 0.5%, B: 0.0001% to 0.0050%, and Cr: 0.01% to 5%, and the balance being Fe and incidental impurities, having a microstructure including, in terms of area ratio, 30% or more of a ferrite phase, and having a tensile strength of 850 MPa or more. Moreover, Patent Literature 3 discloses a galvannealed steel sheet excellent in terms of ductility and corrosion resistance, the steel sheet having a chemical composition containing, by mass %, C: 0.10% to 0.50%, Mn: 1.0% to 3.0%, Si: 0.005% to 2.5%, and Al: 0.005% to 2.5%, in which P content is limited to be 0.05% or less, S content is limited to be 0.02% or less, N content is limited to be 0.006% or less, and the total content of Si and Al satisfies the relationship Si+Al≥0.8%, and having a microstructure including, in terms of area ratio, 10% to 75% of a ferrite phase and 2% to 30% of a retained austenite phase, in which C content in the retained austenite phase is 0.8% to 1.0%.